1. Field of the Invention
The present invention relates to a method and apparatus for processing electrical signals in order to recognize signal features indicative of variations in a process producing the electrical signals, and is particularly of interest for acoustic emissions or stress waves detected by transducers.
2. Description of Related Art
Acoustic emission or stress wave activity is associated with operating machinery or processes, and is produced as a result of friction or impacts taking place during the operation of the machinery or process.
In prior art methods of processing acoustic emissions a transducer detects the acoustic emissions and produces an electrical signal, which corresponds to the acoustic emission activity. In a first method the level of the electrical signal is measured, for example the mean level (RMS level). A good component operating under normal conditions gives a relatively low electrical signal level, corresponding to a relatively low acoustic emission activity whereas a bad component under normal conditions gives a relatively high electrical signal level, corresponding to a relatively high acoustic emission activity. A good component operating under adverse conditions also gives a relatively high electrical signal level. This method is therefore only suitable for use at a fixed operational condition, or some method of normalizing the electrical signal level measurement with respect to the operating conditions is required. As an example, if the acoustic emission activity from a rotating structure increases with rotational speed, it would be necessary to normalize for different rotational speeds. Furthermore, this type of measurement is sensitive to the detection sensitivity of the transducer during the measurement.
In a second method the ratio of the peak level of the electrical signal to the mean level (RMS level) of the electrical signal is measured. The ratio of the peak level of the electrical signal to the mean of the electrical signal provides a measure of variations in the nature or form of the source processes producing the acoustic emissions enabling the occurrence of distress to be detected due to the increased occurrence of transient signal excursions. This method has the advantage of being self normalizing since to a first approximation it is independent of the operating condition and variations in signal detection sensitivity. A disadvantage of this method is the susceptibility of the measurement to spurious electrical noise signals such as those caused by electromagnetic switching transients, which can give rise to relatively high ratios of peak level of electrical signal to mean level of electrical signal.
A third method measures the amount of electrical signal which exceeds a predetermined threshold level. Ideally the threshold level floats dependent upon the electrical signal level viewed over a longer time period. For example the threshold level may be two times the mean level of the electrical signal. This method also has the advantage of being self normalizing. A disadvantage of this method is that as the number of transient signal excursions per unit time increases there is also a tendency for the floating threshold level to rise.
It is possible for this method to give a reducing value for the amount of signal which exceeds the threshold level as the rate of transient signal excursions increases because the floating threshold level rises significantly over and above that caused by any rise in the continuous level.